Internal combustion engines must be cooled due to the fact that the surfaces in contact with hot gases and their lubricants in the interior of the cylinder can only withstand the occurring temperatures to a certain extent without undergoing damage. Individual components, such as spark plugs, injection nozzles, exhaust gas valves, prechambers and piston heads must be able to withstand particularly high mean temperatures such that components of this type must be made of highly heat-resistance materials or be provided with an adequate heat dissipation mechanism and special cooling means.
Consequently, this heat dissipation is effected by a cooling system, in which a cooling fluid flows through cooling water channels that surround at least the cylinders and the cylinder head in order subsequently to discharge the heat at least partially into the surroundings by means of a radiator or to use the heat to heat, for example, a vehicle interior by means of a heat exchanger.
In this context, the term “cooling agent” should be understood as a collective designation for heat transfer media in different coolers that act to dissipate heat, e.g., for cooling motor vehicle engines, nuclear reactors and chemical reaction apparatus, as well as metals being machined (drilling oil, cutting oil).
Cooling agents may be in the form of gases, liquids or solids.
One frequently used coolant is water, to which is mixed anti-freezing agents, hardness stabilizers, corrosion inhibitors (corrosion) etc.
FIG. 6 shows such a cooling system according to the state of the art, i.e., a cooling circuit 60 for cooling a motor vehicle engine 5.
According to FIG. 6, this cooling circuit 60 features a main circuit or main flow path 61 as well as a secondary circuit or secondary flow path 62, through which the coolant flows in the indicated coolant flow direction 8 in coolant lines 67.
In this case, the internal combustion engine 5 to be cooled, a cooling module 2 used for cooling the coolant, in this case a coolant/air cooler (KM/L-K) 2 with functionally indicated blower 3, as well as a coolant pump 6 for moving the coolant through the cooling circuit, are arranged in the main circuit 61 so that a direct connection between the internal combustion engine 5 and the cooling module 2 is produced in the main circuit 61.
Cooling modules known from the state of the art, for example, from DE 100 18 001 A1 and DE 197 31 999 A1, are modules consisting of several heat transfer elements such as coolant coolers, supercharger intercoolers, condensers or oil coolers that are combined into one structural unit or module, for example, into the coolant/air cooler (KM/L-K) 2 according to the described embodiment, and arranged in a cooling circuit.
FIG. 6 also shows that a large circuit 61b in which the coolant flows through the internal combustion engine 5 and through the cooling module 2 during a warm/hot phase of the internal combustion engine 5, as a well as a small circuit in which the coolant only flows through the internal combustion engine 5 during a cold phase of the internal combustion engine 5, are realized within the main circuit 61.
An expansion thermostat 63 arranged between the internal combustion engine 5 and the KM/L-K 2 in the coolant flow direction changes or switches over between the small circuit 61a and the large circuit 61b when the coolant temperature limit is reached.
A compensation tank (AGB) 70 used, among other things, for eliminating gas from the cooling system (degassing) is arranged in the secondary circuit 62. The compensation tank 63 and the secondary circuit 62 are respectively connected to the main circuit 61 by means of auxiliary lines, namely a degassing line KM/L-K 64, through which the secondary coolant flow is conveyed from the KM/L-K 2 to the AGB 70, and an internal combustion engine degassing line 65, through which a secondary coolant flow is conveyed from the internal combustion engine 5 to the AGB 70, as well as a suction line 66, through which a secondary coolant flow (degassed in the AGB 70) is conveyed back into the main circuit or main flow path 61 from the AGB 70.
It is known that such an AGB 70 fulfills the following functions:
a) it ensures an equalization of coolant expansion and a pressure build-up during the heating of the coolant;
b) the cooling system is filled via the AGB;
c) it serves as a reservoir or a reserve volume for the coolant, and
d) the separation of gas from the cooling system (degassing) is realized by the AGB.
This known compensation tank 70 is illustrated separately in FIGS. 7a-c. 
According to FIGS. 7a-c, the AGB 70 consists of a two-part AGB housing 71 that is assembled from a first front housing shell 71a and a second rear housing shell 71b along a vertical plane of partition.
Horizontal and vertical ribs 80 are respectively arranged in the AGB housing 71 and on the interior sides (not visible) of the first housing shell 71a and the second housing shell 71b such that several cuboidal chambers 81 are formed within the AGB housing 71. The coolant flow in the AGB 70 is realized through openings 82 in the ribs 80.
According to FIGS. 7a-c, a filler neck 72 for filling the cooling system 60 (see function b)) is further provided on the upper left corner of the AGB 70. An outlet connector 74 is arranged diagonally opposite of the filler neck 72, i.e., on the lower right corner of the AGB 70, wherein the suction line 66 is connected to the outlet connector and the degassed coolant flows out of the AGB 70 through said outlet connector.
FIGS. 7a-c also show that a pressure relief/suction relief valve 73 is arranged on the AGB 70, approximately in the center of the upper side of the AGB 70, in order to equalize the expansion and build-up of pressure when the coolant is heated (see function a)).
Two ventilating connecting pieces 78, 79 are arranged in the upper region of the right side of the AGB 70, namely a first ventilating connecting piece 78 for the degassing line KM/L-K 64 or connection to it, as well as a second ventilating connecting piece 79 for the internal combustion engine degassing line 65.
A level indicator with a corresponding sensor arrangement 75 is situated underneath the two ventilating connecting pieces 78, 79.
FIG. 7c shows that a silica gel reservoir 76 for mixing or adding corrosion inhibitors (corrosion) to the coolant is arranged in the AGB, particularly on the interior side of the first housing shell 71a, approximately at the center of the shell.
In order to fix the AGB 70 on the cooling module 2 as shown in FIGS. 7a-c, the AGB 70 features two mounting flanges or mounting bolts 70, by means of which the AGB 70 is fixed on a fan cowl or fan housing of the cooling module 2 (not shown).
Here, it should be noted that although it is usually integrated into the cooling module and is included in the parts list, the AGB typically represents an extra component of the cooling circuit.
One disadvantage of this known cooling circuit with a compensation tank arranged in the secondary flow path is the wide variety of required components, for example, several connecting lines and degassing lines that increase the materials outlay and therefore the manufacturing costs, and which may make it impossible to realize a compact design.
In addition, a wide variety of components is usually associated with the increased risk of parts failure.